Corrosion-inhibited phosphate solutions and compositions useful for manufacturing them



3,293,189 Patented Dec. 20, 1966 ice 3,293,189 CORROSION-INHHBITEDPHGSPHATE SOLUTIONS AND CUMPQSHTIONS USEFUL FOR MANUFAC- TURKNG THEMWilliam W. Morgenthaler, St. Ann, Mo., assignor to Monsanto Company, acorporation of Delaware No Drawing. Filed Nov. 6, 1962, Ser. No. 235,86512 Claims. (Cl. 252387) This invention relates to improved aqueoussolutions of phosphate salts, which solutions are characterized byhaving a significantly reduced tendency to corrode copper metal andcopper-containing alloys. More specifically, the present inventionrelates to corrosion-inhibited ammonium phosphate solutions suitable forprolonged use in handling and storage equipment made of copper and/ orcopper alloys.

Aqueous ammonium phosphate solutions have many uses: One of the mostvaluable recent developments in the field of fire fighting, and moreparticularly, forest fire fighting, was that relating to the use ofaqueous solutions of ammonium phosphate salts. Since the presentinvention is particularly valuable when practiced in connection withforest fire fighting, the present discussion will be directed towardthis particular end use. However, it should be kept in mind that theinvention is generally useful for inhibiting corrosion of copper byammonium phosphate solutions. Ordinarily, the solutions (often thickenedwith a viscosity modifying agent so that the solutions stick readily tosurfaces with which they come into contact) are dropped onto brush,trees and dry grass in the path of a fire in order to slow or stop theprogress of the fire. The extensive use of such ammonium phosphatesolutions for forest fire control has been slowed because of the concernby those in the field over the natural corrosivity of such aqueousammonium phosphate solutions toward some of the vital parts ofair-tankers and storage equipment.

While, ordinarily, aqueous solutions containing several weight percentof dissolved ammonium phosphate are not considered to be extremelycorrosive toward copper metal or copper alloys such as brass and bronzeand the like, they are, nevertheless, sufilciently corrosive to causegreat concern when the aqueous solutions are to come into physicalcontact with vital aircraft parts, since failure of the aircraft partsdue even to slow corrosion occurring over a prolonged period of timecould result in the loss of the aircraft and crew. Thus, in order to beconsidered completely acceptable for use in forest fire fighting by theair-drop method, it is necessary that the corrosivity of aqueousammonium phosphate solutions, particularly toward copper metals, besignificantly reduced.

Consequently, it is an object of the present invention to provideaqueous solutions containing ammonium phosphate salts useful for forestfire fighting, the normal tendency of which to corrode copper andcopper-containing alloys is significantly reduced or substantiallyeliminated.

It is another object of the present invention to provide novel methodsof inhibiting the corrosion of copper equipment by aqueous ammoniumphosphate solutions.

It has now been found that the normal or natural tendency of aqueoussolutions of ammonium phosphate salts, such as, for example,monoammonium orthophosphate and diammonium orthophosphate to corrodecopper equipment can be significantly reduced and even practicallyeliminated by the presence (in solution) of an effective amount of aninorganic water-soluble thiocyanate. The term water-soluble inorganicthiocyanate is intended to include all of those inorganic thiocyanatematerials or compounds that are soluble at ambient temperatures in theaqueous ammonium phosphate solutions to be inhibited to at least theextent at which they can discernibly reduce copper corrosion (i.e.,generally at least about 0.005 weight percent). It includes, forexample, the alkali metal and alkaline earth metal thiocyanates such assodium, potassium, lithium, calcium, magnesium and strontiumthiocyanates as well as ammonium thiocyanate. Of these, generallysodium, potassium and/ or ammonium thiocyanates are preferred. Actually,since the protection from corrosion that can be afforded copperequipment by practicing the present invention apparently results fromthe thiocyanate (anion) portion of the thiocyanate compound, theparticular source from which the thiocyanate anions are derived inmanufacturing the aqueous ammonium phosphate solutions is not at allcritical, so long as a suflicient amount of thiocyanate anions areprovided in the phosphate solutions to effectively decrease the normalcorrosivity of the phosphate solutions toward equipment made of copperand copper-containing alloys.

The ammonium phosphate solutions in which these thiocyanate compoundsperform particularly effectively are those that contain at least about0.5 weight percent, and up to the level at about which the solutions aresaturated therewith, but preferably between about 5 and about 25 weightpercent of dissolved ammonium phosphate salts, including monoammoniumorthophosphate and/or diammonium orthophosphate salts, and alsoincluding the diammonium and monoammonium mixed orthophosphate saltsthat also contain an alkali metal cation, such as monoammonium disodiumorthophosphate, monoammonium dipotassium orthophosphate and the like, nomatter from what source these materials were derived, or in what formthe materials are initially introduced into the aqueous compositions.Generally, it is preferred that the inhibited aqueous phosphatesolutions of this invention have a pH between about 3 and about 9. Whenmonoammonium orthophosphate is the only ammonium phosphate salt presentin the inhibited fire-control compositions of this invention, thepreferred pH of these compositions is generally from about 3 to about 5.Similarly, when diammonium orthophosphate is the only ammonium phosphatesalt contained therein, the preferred pH is from about 7 to 9. Whenmixtures of monoammonium orthophosphate and diammonium orthophosphateare utilized, pHs between these preferred ranges are generally moredesirable. However, compositions having pHs somewhat higher or lowerthan these preferred ranges can also be utilized to advantage in thepractice of the present inven-tion.

Although the presence of even a very small amount of the water-solublethiocyanate compounds in the aqueous ammonium phosphate solutionsdescribed above has a corrosion-inhibiting effect upon them, as ageneral rule, aqueous phosphate compositions containing at least about0.01 and preferably at least about 0.04 weight percent of one or more ofthe thiocyanate compounds in the dissolved state should be utilized.Generally optimum corrosion-inhibiting effects can be attained byutilizing at least abut 0.10 weight percent of the thiocyanate compound.Stated otherwise, the corrosion-inhibited aqueous ammoniumorthophosphate compositions of this invention should ordinarily containfrom about 0.075 to about 7.5, and preferably from about 0.03 to about1.5 weight percent of thiocyanate anions dissolved therein.

The corrosion-inhibited ammonium phosphate solutions of the presentinvention can also contain materials other than the phosphate salts andthe thiocyanate compounds without detracting substantially from thebenefits that can be obtained by practicing this invention. For example,the solutions can contain minor amounts of surfactants; inorganic andorganic complexing agents such as the alkali metal tripolyphosphate,pyrophosphates and trimetaphosphates, as well as the higherpolyphosphates such as the hexametaphosphates; and can also containethylenediamine tetraacetic acid and various alkali metal and ammoniumsalts thereof and the alkali metal and ammonium nitrilotriacelates.These compositions can also contain any of the various thickening agentssuch as carboxymethylcellulose, algin, guar gum andcarboxyethylcellulose; other corrosion-inhibiting ingredients; andinorganic and organic pigments and dyes; as Well as many othermaterials.

The corrosion-inhibited phosphate compositions of this invention can bemanufactured via any of a number of methods without any noticeablydetrimental effects upon the ultimate performance of the compositions.For example, the thiocyanate compound can simply be dissolved byintermixing it into or with the otherwise completely formulated aqueouscompositions, or at any other stage during the preparation of thefire-control composition. Via another method, the thiocyanate compoundcan first be dissolved in water and the resulting solution thenintermixed subsequently with the ammonium phosphate materials.

Still another process for manufacturing the aqueous corrosion-inhibitedphosphate compositions described heretofore involves one of thepreferred embodiments of the present invention. This preferredembodiment c-omprises preparing an intermediate concentrate mixture ofone or more of the ammonium phosphate salts described heretofore withone or more of the thiocyanate (inhibitor) compounds, generally in aparticulated, solid form. Preferably the inhibitor compound(s) andammonium phosphate salt(s) will be present in this concentratecomposition or mixture in amounts in direct proportion to theirrespective amounts found in the fire-control compositions describedheretofore, so that the final fire-control composition can bemanufactured by simply dissolving the concentrate in a sufiicient amountof water. In other words, generally the preferred concentratecompositions of this invention will contain at least one of theabove-described ammonium phosphate salts and at least one of thethiocyanate (inhibitor) compounds in a weight ratio of phosphate salt toinhibitor compound, respectively, of from about 3800z1 to about :1, andpreferably from about 750:1 to about 100:1. It is still furtherpreferred that these particulated, solid concentrate compositionscontain a total of at least about 50 weight percent of one or more ofthe above-described ammonium orthophosphate salts. These preferredconcentrate compositions can also contain effective amounts ofpractically any other ingredients that can be utilized in fire-fightingcompositions, including a thickener if one is desired in the finalfire-control compositions, as well as dyes, pigments, water-softeningagents, and the like, all of which should preferably be present in theconcentrate compositions in minor amounts, as compared to the ammoniumphosphate salt(s) contained therein. Some of the advantages of thesepreferred concentrate compositions can readily be appreciated when it isrealized that at most points from which aircraft are loaded for theirvarious air-drops of fire-control compositions, very little weighing andhandling equipment is available that is capable of manufacturing thefire-control compositions from the individual components. Thus, in theabsence of the preferred concentrate compositions described above (whichconcentrate compositions need simply be dissolved in appropriate amountof water in order to be converted into the final, valuable fire-controlcompositions of the present invention), as a practical matter, suchcorrosion inhibited final aqueous fire-control compositions as thosedescribed heretofore could not be utilized.

Typical examples of the preferred concentrate compositions of thisinvention include:

(a) 99.0 weight percent of diammonium orthophosphate and 1.0 weightpercent of sodium thiocyanate.

(b) 90.0 weight percent of monoammonium orthophosphate, 1.0 weightpercent of ammonium thiocyanate and 9.0 weight percent of sodiumalginate.

(c) 90 weight percent of diammonium orthophosphate, 0.5 weight percentof potassium thiocyanate and 9.5 weight percent of sodiumcarboxymethylcellulose.

(d) 55 weight percent of an equimolar mixture of monoammoniumorth-ophosphate and diammonium orthophosphate, 1.0 weight percent ofammonium thiocyanate, 35 weight percent of sodium tripolyphosphate and 9weight per-cent of sodium alginate.

(e) 89 weight percent of monoammonium orthophosphate, 0.25 weightpercent of magnesium thiocyanate and 10.75 weight percent of pentasodiumaminotrimethylenephosphonate.

(f) 70 weight percent of diammonium orthophosphate, 20 weight percent ofmonoammonium orthophosphate, 9 weight percent of sodiumcarboxyethylcellulose and 1 weight percent of sodium thiocyanate.

(g) 84 weight percent of diammonium orthophosphate, 15.50 weight percentof guar gum and 0.50 weight percent of ammonium thiocyanate.

These concentrate compositions are also useful as corrosion-inhibitedde-icer compounds, wherein, either alone or in combination withadditional corrosion inhibiting materials, they can be utilized in amanner similar to that in which more corrosive, less desirable, sodiumchloride is presently conventionally utilized. For example, theseparticulated, solid ammonium phosphate-thiocyanate compounds caneffectively de-ice sidewalks, roadways and the like when they are simplyspread over ice and/ or snow.

The concentrate compositions can be prepared via any of a number ofconvenient procedures, including simply blending or mixing together theappropriate amounts of the various dry ingredients. Although the size ofthe individual particles in these preferred concentrate compositions isnot critical in so far as the practice of the present invention isconcerned, it is generally preferred that they be of sufiiciently smallsize to pass through a US. Standard 12 mesh screen. For optimum results,at least about weight percent of these particles should be small enoughto pass through a US. Standard 40 mesh screen. Although commericalgrades of the various available phosphate and soluble thiocyanate saltsdo not necessarily meet these optimum standards of particle size, theycan readily be utilized in the preparation of the compositions of thisinvention.

In the following examples, which are illustrative of some of thepreferred embodiments of the present invention, all parts are by weightunless otherwise specified.

Example 1 Into a conventional aluminum storage tank fitted with a fairlyefiicient mixer are charged 8.34 parts of water and 1.2 parts of apre-prepared blend made up of 84 parts of crystalline technical gradediammonium orthophosphate, 15 parts of sodium alginate and 1 part ofsodium thiocyanate. The resulting mixture is then stirred until all ofthe diammonium orthophosphate has dissolved.

Into the resulting fire-control composition are immersed several clean,preweight soft copper (1 x 2" x coupons. The coupons are continuouslyimmersed and removed from the composition in a cycle adjusted to that R.I the coupons remain immersed in the composition for 30 seconds, andthen remain out of the solution, exposed to the ambient air, for 30seconds. This procedure is continued for a total of 72 hours, afterwhich the coupons are removed from the fire-control composition, rinsedwith distilled water and acetone and then reweighed. The loss in weight(in milligrams) is then appropriately inserted into the equation:

534W orrosion 111 m1 s per year wherein:

W weight lost during test in milligrams, D=specific gravity of metal, Aexposed surface area in square inches,

-T=time of exposure to solution in hours,

in order to determine the corrosion that has taken place, expressed interms of mils of penetration per year. The corrosion rate or rate ofattack on copper by the composition of Example I is thus found to beabout 1.0 mils per year.

By comparison, a composition which is prepared in the same way as thatin Example I, above, but without the sodium thiocyanate, corrodessimilar copper coupons at a rate of about 1,000 mils per year.

In a manner similar to that just described, other compositionscontaining various levels of inorganic thiocyanate compounds are tested.Results of these tests are given in Table 1, below:

and mixtures thereof, and from about 0.075 toabout 7.5 weight percent ofdissolved thiocyanate ions.

4. A corrosion-inhibited liquid aqueous ammonium phosphate compositionconsisting essentially of water from about 5 to about 25 weight percentof an ammonium orthophosphate salt and from about 0.03 to about 1.5weight percent of dissolved thiocyanate ions.

5. A concentrated ammonium phosphate composition suitable for use in themanufacture of aqueous corrosioninhibited ammonium orthophosphatefirefighting solutions, said composition consisting essentially of atleast one ammonium orthophosphate salt selected from the groupconsisting of monoammonium orthophosphate and diammonium orthophosphateand a water-soluble inorganic thiocyanate compound; the weight ratio ofsaid ammonium orthophosphate salt'to'said thiocyanate com pound in saidcomposition being from about 3800:1 to about 10:1

6. A particulated solid concentrate composition consisting essentiallyof at least about weight percent of an ammonium orthophosphate materialselected from the group consisting of monoammonium orthophosphate,diammonium orthophosphate and mixtures thereof, and an inorganicthiocyanate compound selected from the group consisting of ammonium,alkali metal and alkaline earth metal thiocyanates; the weight ratio ofsaid ammonium orthophosphate material to said thiocyanate compound insaid concentrate composition being from about 750:1 to about 100:1.

TABLE 1.CORROSION INHIBITED AMMONIUM PHOSPHATE SOLUTIONS ExampleCorrosion No. Ammonium Phosphate Parts Thiekener 1 Inhibitor Parts MetalRate Control Diammonium Orthophosphate 10 Copper 1, 500

Do do 10 Brass... .3

10 Bronze 1, 300 10 Copper 940 Do Monoammoniurn orthophosphate... 10 15II Diammoniurn Orthophosphate 5 0.05 1 III d 5 0.05 2 IV 5 0.05 2 V 5Ammonium Thiocyanate.. 0. 04 1 VI 10 Sodium Thioeyanate 0.10 1 VII 10CMC Potassium Thioeyanate- 0. 05 6 VIII 10 CMC Magnesium Thiocyanate0.05 6 IX. 10 CMC Ammonium Thioeyanate. 0. 10 Bronze... 2 X onoamm niumOrthophospha 1O CMC Sodium Thiocyanate 0.05 Copper. 0 XI {DiammoniumOrthophosphate. 5 0 05 do 1 Monoammonium Orthophospha 5 Diammoniumorthophosphate. 10 XII Monoamrnonium orthophosphate 1 0.05 .do 5

Tetrasodium Pyrophosphate 3 XIII Diammonium orthophosphate 5 0. 10 do 02 Carboxyrnethylcellulose.

What is claimed is:

1. An aqueous ammonium phosphate composition consisting essentially of,in addition to at least one inorganic ammonium orthophosphate selectedfrom the group consisting of monoammonium orthophosphate and diammoniumorthophosphate, an inorganic water-soluble thiocyanate compound; therebeing dissolved in said compositions from about 0.5 to about 25 weightpercent of said inorganic ammonium orthophosphate, and from about 0.03to about 7.5 weight percent of dissolved thiocyanate 1011.

2. An aqueous composition as in claim 1, wherein said inorganicwater-soluble thiocyanate compound is selected from the group consistingof ammonium, alkali metal and alkaline earth metal thiocyanates.

3. A corrosion-inhibited liquid aqueous ammonium phosphate compositionconsisting essentially of at least about 0.5 weight percent of anammonium orthophosphate salt selected from the group consisting ofmonoammonium orthophosphate, diammonium orthophosphate copper alloysupon contact with a normally corrosive aqueous ammonium orthophosphatesolution consisting essentially of water and, dissolved therein, fromabout 0.5 to about 25 weight percent of ammonium orthophosphate 7selected from the group consisting of monoammonium orthophosphate,diammonium orthophosphate and mixtures thereof, which method compriseseffecting said contact when said aqueous ammonium orthophosphatesolution contains dissolved therein from about 0.075 to about 7.5 weightpercent of thiocyanate ions.

References Cited by the Examiner UNITED STATES PATENTS Re. 25,394 6/1963Martinson 71-34 1,997,669 4/ 1935 Arcieri. 2,215,077 9/1940 Beekhuis eta1. 252--387 2,220,059 11/1940 Beekhuis et a1. 252387 2,238,651 4/1941Keenen 252387 2,303,399 12/1942 Schwartz 252387 2,636,009. 4/1953 Irwin2,901,428 8/1959 Schulenburg FOREIGN PATENTS 5 575,798 5/1959 Canada.

OTHER REFERENCES Chemical Week, v01. 89, No. 14, Oct. 17, 1961, pagesSAMUEL H. BLECH, Primary Examiner.

JULIUS GREENWALD, ALBERT T. MEYERS,

Examiners.

1. AN AQUEOUS AMMONIUM PHOSPHATE COMPOSITION CONSISTING ESSENTIALLY OF,IN ADDITION TO AT LAST ONE INORGANIC AMMONIUM ORTHOPHOSPHATE SELECTEDFROM THE GROUP CONSISTING OF MONOAMMONIUM ORTHOPHOSPHATE AND DIAMMONIUMORTHOPHOSPHATE, AN INORTANIC WATER-SOLUBLE THIOCYANATE COMPOUND; THEREBEING DISSOLVED IN SAID COMPOSITIONS FROM ABOUT 0.5 TO ABOUT 25 WEIGHTPERCENT OF SAID INORGANIC AMMONIUM ORTHOPHOSPHATE, AND FROM ABOUT 0.03TO ABOUT 7.5 WEIGHT PERCENT OF DISSOLVED THIOCYANATE ION.
 12. A METHODOF RETARDING THE CORROSION OF COPPER AND COPPER ALLOYS UPON CONTACT WITHA NORMALLY CORROSIVE AQUEOUS AMMONIUM ORTHOPHOSPHATE SOLUTION CONSISTINGESSENTIALLY OF WATER AND, DISSOLVED THEREIN, FROM ABOUT 0.5 TO ABOUT 25WEIGHT PERCENT OF AMMONIUM ORTHOPHOSPHATE SELECTED FROM THE GROUPCONSISTING OF MONOMMONIUM ORTHOPHOSPHATE, DIAMMONIUM ORTHOPHOSPHATE ANDMIXTURES THEREOF, WHICH METHOD COMPRISES EFFECTING SAID CONTACT WHENSAID AQUEOUS AMMONIUM ORTHOPHOSPHATE SOLUTION CONTAINS DISSOLVED THEREONFROM ABOUT 0.075 TO ABOUT 7.5 WEIGHT PERCENT OF THIOCYANATE IONS.